Currently, our system does not allow you to add seats at the time of renewal, just before and after. However, any seats that you have added ahead of time will be part of your renewal. That is if you purchased 10 seats in January and another 5 in March, your renewal will be for a total of 15.
We recommend you do not add seats right before your renewal because you will be charged a full 6 months of access even if there is only 1 day remaining in your contract (we only prorate daily for added seats up until month 6 into the contract, after that, any added seat still costs half of a yearly license).
We instead recommend that you wait until your renewal has processed and add new seats the next day, that way your seats will start at about the same time and you will be charged for the new year only.